The glass manufacturing process generally takes place through a succession of related steps that are carried out at separate stations in an overall glass furnace; each of the stations generally has correspondence to one of the manufacturing steps. More specifically, the usual glass manufacturing process includes the steps of drawing a sheet of glass from a bath of molten glass, conveying the molten glass sheet through an annealing furnace with a decreasing temperature gradient over its length that allows the sheet to cool slowly to prevent the buildup of compressive forces within the glass; cutting the annealed glass sheet to the shape and size desired; and tempering the cut sheet by a heating and sudden cooling process to give the glass sheets high compressive forces at their surfaces to minimize susceptibility to breakage and shattering.
The glass material must be conveyed through each of the successive stations at a controlled rate. One known means for conveying the glass sheets between and within a station is to provide each station with a conveyor mechanism defined by a pair of cooperative conveyor drives spaced apart from one another at opposite sides of the station; each of the conveyor drives being defined by a continuous drive loop framed over a pair of cooperative pulleys. A plurality of elongate, cylindrical rollers are spaced apart from one another with respect to the direction of glass movement, and have their opposed ends supported on the respective conveyor drives in frictional engagement therewith. The cooperative rotation of the conveyor drives imparts rotational motion to the rollers. A glass sheet carried on the rollers will be transported through the station in accordance with the net driving torque applied to the conveyor drives.
In an earlier practical design of a glass conveyor mechanism of this type, predating the glass conveyor mechanism of the present invention, the continuous drive loop of each conveyor drive was defined by an endless steel band. The band was formed from a long strip of stainless steel having a thickness of approximately 1/16 inch. The stainless steel strip was made into a loop by welding the opposed ends together to form a smooth seam. This form of continuous drive loop required special design attention and periodic maintenance. Specifically, the use of the band in a relatively long station that required high levels of driving torque to transport the glass through the station, caused relatively high tension in the band as it passed over the conveyor drive pulley. In addition, relatively large, special pulleys were needed to drive the band to minimize metal fatique caused by the bending of the band when it conformed to the shape of the conveyor pulleys. Maintenance of this drive system included the rewelding of a belt that became separated at the seam, and periodic replacement. Experience with such drive systems provided the stimulus for the improved conveyor mechanism of the present invention.
More factually, a Model F-400 furnace built by Glasstech, Inc. and which employed the earlier endless, stainless steel band drive loop was placed in a commercial installation in February of 1974. In May of 1974, the stainless steel band drive loop of that furnace was replaced by an alternative drive loop design. First the stainless steel band and flat pulley arrangement were replaced by a flat-top steel drive chain and sprocketed pulley arrangement. This design change revealed a characteristic peculiar to the chain drive loop. Specifically, when the chain was being accelerated or decelerated through the range of zero to one-third of its normal operating speed, it would vibrate. These vibrations were traced to the slip-stick friction phenomenon that frequently occurs in mechanical drive systems. More precisely, the chain was experiencing a transition between the effects of static friction and kinetic friction in this low speed range, i.e. one chain link may be subject to static frictional effects, while at the same time another chain link may be subject to kinetic frictional effects. The slip-stick friction phenomenon manifests itself as vibration in the moving chain. However, because the vibratory effects were only present when the chain was being driven at below one-third the normal chain velocity, they did not prohibit the use of the chain design.
Another characteristic of the chain drive design came to light when observing the operation of a Model F-501 furnace that was installed at a site in August, 1975. A Model F-501 is an oscillating roller-hearth furnace that oscillates a glass sheet load within the furnace by alternately driving the chain sprocket forward and backward in contrast to the Model F-400 which is a continuous motion furnace. With each reversal in the driving torque applied to the drive sprocket there would be an appreciable amount of lost motion caused by the slack present in the drive chain. This lost motion effect would allow the rollers to be momentarily stopped even though the sprocket driving the chain was moving smoothly.
At this juncture changes were explored to modify the chain drive design in such a manner that the vibration and lost motion would be eliminated.
Under the design developed, as will hereinafter be more fully set forth in the Disclosure of the Invention and Best Mode for Carrying out the Invention, a minimum level of tension is maintained on the active area of the chain irrespective of the direction of chain travel. This broad idea was implemented on an experimental basis in a Model F-502 furnace installed in June, 1976 in Toledo, Ohio and incorporated as a standard feature on a Model F-503 furnace installed in July 1977 in South Africa. The experimentation begun in June, 1976, culminated in the improved conveyor mechanism design forming the subject of this patent application.